Lets see....the dictionary's definition of crazy is:

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crazy adjective
cra·​zy | \ˈkrā-zē \
crazier; craziest
Definition of crazy (Entry 1 of 3)
1a : full of cracks or flaws : UNSOUND
… they were very crazy, wretched cabins …
— Charles Dickens
b : CROOKED, ASKEW
2a : not mentally sound : marked by thought or action that lacks reason : INSANE sense 1b
yelling like a crazy man
—not used technically
b(1) : IMPRACTICAL
a crazy plan

(2) : ERRATIC
crazy drivers

c : being out of the ordinary : UNUSUAL
a taste for crazy hats
3a : distracted with desire or excitement
a thrill-crazy mob
The fans went crazy when their team won the championship.
b : absurdly fond : INFATUATED
He's crazy about the girl.
c : passionately preoccupied : OBSESSED
crazy about boats

It doesn't matter the cause it is the behavior that is defined. So if a MI person or someone without a MI comes across as thinking or acting in a way that lacks reason.....it fits the definition so that is exactly why the term is used. A lot of people fit the definition & many MI behavior & thinking manifest themselves in a way that fits the definition too.

I have been there even with my depression & I admit that my thinking & actions were crazy. I don't see the problem that when something fits a definition that the definition should not be used?